The magnetic core has the effect of accelerating induction heating by concentrating lines of magnetic force on a workpiece and increasing the power of the coil in the case where the magnetic core is mounted on a rear surface of a coil and has the effect of preventing a portion not required to be hardened from being heated by shielding the lines of magnetic force in the case where the magnetic core is mounted on a front surface of the coil. Thus the magnetic core is a component part indispensable for the heating coil of the high frequency hardening apparatus
For example, in the case where the workpiece to be subjected to high frequency hardening has a complicated configuration which necessitates a hardening depth to be adjusted, it is possible to change the state of the induction heating and control the hardening depth of the workpiece by altering the configuration, size, number, direction, and position of the core to be mounted on the heating coil. The material for the core is required to have (1) a satisfactory frequency characteristic, namely, to have a small change in the frequency change-caused inductance of the core, (2) a high saturation magnetic flux density, (3) a high relative permeability, and (4) a low iron loss.
To adapt the magnetic core to various configurations of the workpiece, it is often the case that parts of the core are produced in small lot production of many products. Thus in many cases, parts of the core are produced one by one by cutting work. Therefore materials for the core are demanded to have high strength and cutting workability.
Because powder-metallurgy processing is capable of producing the magnetic core with a low of raw materials and excellent in mass-productivity, the magnetic core produced by the powder-metallurgy processing is frequently used for the heating coil of the high frequency hardening apparatus.
As the magnetic core for an high frequency hardening coil, Fluxtrol A (trade name, produced by Fluxtrol Inc.) composed of iron particles fixed to one another with fluororesin and Poly-iron (trade name, produced by NEC Tokin Corporation) composed of sendust particles fixed to one another with phenol resin have been used. These magnetic cores have problems that the materials for the magnetic cores have a comparatively low strength, crack when a thin portion is cut, and are broken in mounting the magnetic cores on the coil.
As the magnetic core for use in an electric motor or a reactor, there is known the method for producing the powder magnetic core by mixing the magnetic powder having the insulation films formed on the surface of the pure iron powder thereof in advance and the silicon resin powder with each other, gelling the resin powder in the predetermined temperature atmosphere, and compression molding (warm molding) the mixture of the magnetic powder and the resin powder (patent document 1).
There is known the method for producing an oil-impregnated bearing made of iron by mixing the thermosetting epoxy resin with the reduced iron powder to such an extent that the porosity of the reduced iron powder is not reduced to a high extent, coating the surface of the reduced iron powder with the thermosetting epoxy resin, subjecting the mixture to compression molding, hardening, and impregnating the obtained bearing with oil (patent document 2).